The phase difference between the horizontal and vertical synchronizing signals and video signals produced from signal sources such as computers generally vary according to the signal source. The number of picture elements per horizontal scanning period and the number of lines per vertical scanning period of the video signal produced from the signal source also vary.
To enable the display of a wide variety of signals as mentioned above on a video display device, display elements for one picture element of the signal source are conventionally displayed having a picture element : display element ratio of 1 : 1 or 1: integer.
If the number of picture elements in the signal source is less than the number of display elements of the video display device and the picture elements are displayed in a one to one ratio, the display width of that signal source becomes smaller than the displayable screen area. Similarly, if the number of picture elements in the signal source is less than the number of display elements of a video display device and the picture elements are displayed in a one to integer ratio, the image width may become wider than the displayable screen area.
As described above, the prior art may display the video image of the signal source at a narrower width than the displayable screen area of the video display device depending on the video output signal from the computer when video output signals from different models of computer are displayed in a one to one ratio (the ratio of the number of picture elements in the signal source to the number of display elements).
In addition, the video image of the signal source may become wider than the displayable screen area of the video display device in the prior art, depending on the video output signal from the computer, when video output signals from different models of computers are displayed in a one to integer ratio (the ratio of the number of picture elements in the signal source to the number of display elements). As a result, the user may not be able to see part of the video image, and may need to adjust one or both of the horizontal and vertical screen positions to see the missing portion of the video image.